This invention relates to apparatus for applying liquid developer to the surface of a recording medium moving in a path relative to the apparatus and more particularly to liquid developer apparatus employing a single developer roll for providing the dual functions of forming a development apex for developing a latent electrographic image on the surface of the recording medium and a wiping action to remove excess liquid developer from and dry the surface of the developed recording medium.
The art of liquid toning or development of a previously formed electrographic latent image formed on the image bearing surface of a recordingmedium with a liquid toner or developer is a well established art. Typical of the present state of the art are U.S. Pat. Nos. 4,127,082 and 4,454,833. In the first of these patents, the applicator roll for the developer is partly submerged in a liquid developer bath with the roll rotated at a sufficiently high velocity to raise the developer from the bath in the form of a thin film on the surface of the roll due to viscous friction for delivery to to a development gap formed between the applicator roll and the surface of the recording medium. In the second of these patents, the liquid developer is supplied to a region of the rotated applicator roll with flow controlling means positioned in spaced relation to the surface of the applicator to meter the amount of developer applied to and carried on the roll to a development gap formed between the applicator roll and the surface of the recording medium. In both of these types of developing apparatus, a printing gap is established between the surface of the roll and the recording medium surface for delivery of toning liquid and development of the recording medium is followed engagement by the recording medium with a second roll rotated in the opposite direction relative to the applicator roll for wiping excess liquid developer from the medium surface and drying the same. Two rolls may be employed to provide a single development gap or or a series of development gaps, such as disclosed in U.S. Pat. Nos. 3,203,395, 3,256,855 and 4,141,317. The applicator roll may be rotated in a direction opposite to the the direction of movement of the recording medium, such as disclose in U.S. Pat. No. 3,367,791 and UK Pat. No. 1,384,268 and may have a bias applied the development gap to accomplish the developing action, such as disclosed in UK Pat. No. 1,384,268 and U.S. Pat. No. 3,560,204.
The present method of liquid development of recording media, particularly in the electrographic printer/plotter art, is the use of a series combination of an applicator roll and dry or wipe roll as illustrated in the first mentioned patents above as well as in the Benson Electrographic Printer/plotter Model 9336, manufactured by Benson, P.O. Box 32059, 2600 Orchard Parkway, San Jose, CA 95152. Due to high speed movement of the recording medium to reduce printing time and the necessity to print multiple colors by successive passage of the same section of the recording medium over several different color developing apparatus, it is important to immediately wipe excess developer off of the medium surface and invoke medium surface drying to insure that its image bearing surface will not become smeared on the next pass of the medium for latent image writing and subsequent development as well as prevent electrographic head damage caused by electrode shorting due to a wet medium surface.
While these known liquid developing apparatus may have good application for applying liquid developer to the image bearing surface of the recording medium, it would be desirable to eliminate the need of using a second wiping and drying roll by combining the wipe/dry function and developing function into a single roll capability. This would have the advantage of reducing the cost of the apparatus by eliminating the need for the second roll and its drive means (motor and coupling arrangement). Also, the length of the machine bed could be reduced since there would not be two roll in series along the path of movement of the recording medium. However, the elimination of the second wiper/dry roll has been thought not possible since with the establishment of the development gap between the applicator roll and the medium surface, how is one to apply liquid developer into the region of the gap and concurrently wipe excess developer from the same medium surface to promote the drying of its surface?
Another problem is the elimination of background stains when the recording medium is momentarily stopped during application of liquid developer to the the image bearing surface. While the source of developer may be stopped when medium is stopped, the meniscus formed at the development gap will still remain for a short period of time and continue to tone the medium surface at the point of the remaining meniscus. What is needed is some means by which this meniscus is immediately broken and removed upon cessation of the liquid developer supply.
These problems are substantially eliminated by the liquid developing apparatus of this invention.